Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. Chemical energy is the difference between the energy of the starting chemicals and the energy of the product chemicals. A reaction is a process whereby chemicals are transformed from initial chemicals to product chemicals while maintaining the same overall mass. In fuel cells, the reaction at the anode is an oxidation reaction and the reaction at the cathode is a reduction reaction. An oxidation reaction involves the loss of electrons while the gain of electrons is a reduction reaction. A fuel cell is a galvanic cell. A galvanic cell is a device for extracting the energy of a reaction as electrical work. The maximum electrical work is equal to the change in the free energy that occurs when the reactions take place. For example the standard free energy of the reaction2H2(gas)+O2(gas)→2H2O (liquid)is the change in free energy when 2 moles of pure hydrogen at 1 bar reacts with 1 mole of pure oxygen gas at 1 bar to produce liquid water at the same pressure. The standard free energy change for this reaction is −474.26 kilojoules. That is how much energy is available for non-expansion work, which includes electrical work. A liquid is a condensed phase. The free energy change for a fuel cell reaction such as the formation of water from hydrogen and oxygen is related to the cell potential by the following reaction:ΔG=nFE
where n is the amount of electrons (in moles) transferred between the electrodes when the stoichiometric reaction occurs. F is Faraday's constant of 96500 coulombs per mole and E is the cell voltage. The electrical work that is done when charge travels from the anode to the cathode is equal to the product of the cell voltage and the current driven multiplied by the time of the current draw.
A battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the chemical reactants are consumed (i.e., discharged). In a secondary battery, recharging regenerates the reactants, which involves putting energy into the battery from an external source. The fuel cell, on the other hand, is an energy conversion device that theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes. In reality, degradation, primarily corrosion, or malfunction of components limits the practical operating life of fuel cells.
Fuel cells have provided power for space shuttles for a couple of decades. However, the fuel used in the fuel cells used on the space shuttle is pure, liquid hydrogen.
In a conventional fuel cell the electrolyte is catalyzed on both faces of a two dimensional membrane. One face is the anode side where fuel is oxidized and the opposite face is the cathode side where oxygen is reduced. In polymer electrolyte fuel cells, this three-layer system is commonly referred to as a membrane electrode assembly (MEA). The three layers are the polymer electrolyte sandwiched between two catalytic layers.
Gasoline, diesel, methane and alcohols do not have adequate electrochemical reactivity to be used directly in state-of-the-art polymer electrolyte fuel cells (PEFCs) for high power applications. A catalytic-chemical fuel processor is required to convert these fuels to hydrogen-rich fuel gases. Fuel processors for automotive fuel cell engines must be able to start up quickly, follow demand rapidly, and operate efficiently over a wide range of conversion rates. Also, fuel conversion needs to be essentially complete over the entire load range. The carbon monoxide level in the processed fuel entering the stack must be very low to avoid poisoning of the anode electrocatalysts.
The chemical reactions governing fuel processor design for two alternative reactions used in the primary steps to convert methanol, i.e., H3COH, or gasoline (e.g., H3C(CH2)6CH3), are given below:
Steam reforming
(1) 2 H3COH+H2O (steam)+heat→5 H2+CO+CO2 
(2) H3C(CH2)6 CH3+12H2O (steam)+heat→21 H2+4 CO+4 CO2 
Partial oxidation
(3) 2 H3COH+O2 (air)→3 H2+CO+C02+H2O+heat
(4) H3C (CH2)6 CH3+7½O2 (air)→6 H2+4 CO+4 CO2+3 H2O+heat
The steam reforming reactions described in the above equations require catalysts. The catalysts are incorporated into the catalytic-chemical-fuel processor. For the purposes of this patent application, a reforming catalyst is any catalyst that increases the rate of hydrogen formation. This would include the water-gas-shift catalysts that convert CO and water to hydrogen and CO2. Reaction (1) can be considered a combination of methanol cracking and water-gas-shifting. Reaction one can be broken down into (1A) and (1B).
(1A) 2 H3COH→2 CO+4H2 
(1B) 2 CO+2 H2O→2 CO2+2 H2 
However in practical systems the second reaction does not go to completion. That is why the “reformate” fuel is contaminated with CO. Both (1A) and (1B) require catalysts. (1A) is a methanol cracking reaction while (1B) is the water-gas-shift reaction. Thus for the conversion of methanol to hydrogen, catalysts that activate the cracking reaction, the water-gas-shift catalysts, or dual function catalysts that enable both reactions are all referred to as reforming catalysts. These catalysts can be incorporated into a separate reactor external to a fuel cell or be incorporated in the fuel cell itself. When a catalyst is incorporated in the fuel cell, we refer to this as internal reforming. A reactor or set of reactors that chemically changes a fuel to hydrogen is also called a Syngas generator of reactor. There are always side products produced including CO2 and CO. Other side products may also be produced.
The H2 content in reformed methanol or gasoline is about 0.189 kg or 0.430 kg H2/kg fuel respectively. The reforming process yields H2 diluted with CO2, and low levels of CO. Within the operating temperature (T) range of polymer electrolyte fuel cells, the reformate prior to the water gas shift (WGS) and the preferential oxidation (PROX) reactor contains CO at the pph level, enough to shut down a Pt alloy catalyst. The WGS output contains about 1% CO, still enough to shut down the anode. A PROX unit is used to further reduce the CO content to the approximately 10-ppm tolerance limit of a typical anode catalyst (PtRu). The development of CO tolerant anodes could obviate the need for the PROX and WGS units. Today, there are no anode catalysts that could tolerate the 1% CO content of the WGS reactor output at the operating temperature of a polymer electrolyte fuel cell. The most commonly used anode catalyst is carbon supported PtRu. Alternatives such as PtMo have been studied although stability issues with PtMo require examination. Thus the quest for better catalysts should be augmented by the search for higher temperature electrolyte (HTE) systems. Phosphoric acid fuel cells (PAFCs) do not require a PROX unit because they operate at 200° C. Although CO is not a fuel at 200° C., it is not a poison. However, PAFCs suffer from corrosion problems linked to the phosphoric acid electrolyte.
Other reasons exist for increasing the FC operating temperature, even when using pure H2. At high efficiencies (high cell voltage), the polarization at the anode could be less than 30 mV, yet the cell voltage is hundreds of mV off the thermodynamic value because of cathode polarization (or cathode losses). The four-electron oxygen reduction kinetics would improve significantly if high temperature electrolyte systems were developed. Thus, there are 2 key reasons for developing high temperature electrolyte systems (1) mitigation of CO poisoning and (2) improvements in O2 reduction kinetics. High temperature electrolyte systems would substantially reduce the fuel processor/FC system volume (PROX unit could be eliminated and the water gas shifter could be reduced in size or possibly eliminated). Successful development of high temperature electrolytes would afford relaxation of the output requirements of fuel reformers. The application of such a membrane electrolyte assembly (MEA) at even higher temperatures would have an enormous impact on the design of compact systems for portable, transportation and stationary power.
Solid-state proton conductors are candidates in sensors, batteries, fuel cells, electrolysers, etc. A brief overview of the types and principles of solid state proton conductors and the temperature dependence of those conductors is provided by Norby in “Solid-state prototic conductors: principles, properties, progress and prospects,” Solid State Ionics, 125, p.1-11 (1999), which is incorporated herein by reference. Norby states in the Abstract that there is “much needed development of electrodes for high- and intermediate-temperature proton conductors.” The above-mentioned issue of Solid State Ionics is the Proceedings of the 9th International Conference on Solid State Proton conductors. Included in the proceedings are over 50 articles devoted to solid-state proton conductors.
The state-of-the-art polymer electrolyte fuel cells generally use proton-conducting polymers as the electrolyte membrane in the MEA. The state-of-the art proton conducting polymers are low-temperature conductors operating below 100° C. On the other hand, intermediate and high-temperature proton conductors operate in the temperature ranges of 100-650° C. and 650° C. and more, respectively.
The MEA is the core of the fuel cell. Proton conducting polymers are used as the electrolyte in PEFCs. Nafion™ is an example of a proton-conducting polymer. Although electrolytes for fuel cells are necessarily proton conductors, they do not conduct electrons (i.e. they are electronic insulators). Thus Nafion is an example of an electronically insulating proton conductor (EIPC). An EIPC is a material that conducts protons or hydrogen but does not conduct electrons. PEFCs typically operate at temperatures below 100° C. The upper limit is imposed by properties of the polymer electrolyte (typically perfluorinated sulfonated polymers such as Nafion™) that dehydrates at temperatures above 100° C. The maintenance of water (water management) in the polymer is a requirement for high proton conductivity. As the temperature is increased above the boiling point of water, the polymer membrane dehydrates. The dehydration of the membrane reduces the conductivity.
In state-of-the-art PEFCS, the conductivity decreases as the temperature increases. Specifically, the proton conductivity of Nafion™ decreases as the temperature increases. Typically the thickness of Nafion used in fuel cells is between 2 and 7 mil, where a mil is a thousandth of an inch. Such thick films are free standing films. It is possible to maintain hydration of the membrane by increasing the pressure. Increasing the pressure is a way of effecting water management because the boiling point of water increases with increasing pressure. However, operating a fuel cell at higher pressure requires the use of parasitic energy to operate compressors. Maintenance of higher pressure would reduce the power density of the fuel cell system.
FIG. 1 of the specification is FIG. 1 of Norby. It shows selected literature data for proton conductivity as a function of inverse temperature. Norby states that many classes of proton conductors are represented by members that have protonic conductivities of up to 10−3−10−2 S/cm at some temperature. The proton conductivities shown in FIG. 1 for Nafion, HCl and H3PO4 solutions are the ranges where these materials are useful as proton conductors.
There are several disadvantages of the Nafion, HCl and H3PO4 containing proton conductors. Nafion requires water. Phosphoric acid fuel cells have a matrix, e.g., doped polybenzimidazole (PBI) imbibed with phosphoric acid. HCl and H3PO4 are acids and, therefore, corrosive. The operating temperature of those acids is also limited by their volatility. Also, the anion (or conjugate base) of phosphoric acid (i.e. phosphate) poisons the platinum cathode catalyst.
Norby states that “at higher temperatures the protonic conductivities decrease because of (1) reversible or irreversible loss of vehicle water (e.g., in proton conducting polymers), (2) because of decomposition or melting of hydrates, hydroxides or acid salts, or (3) because of reversible loss of protons (water) from oxides. Thus, proton conductivities are generally functional over relatively narrow temperature ranges.”
The units of conductivity are S/cm where S is Siemens. A Siemen is the reciprical of an ohm (i.e. 1/Ω). Referring to FIG. 1, Norby concludes that, “at present, solid proton conductors do not parallel the best oxygen ion conductors (with conductivities>1 S/cm). However, proton conductors, in general, work at substantially lower temperatures and may offer the highest conductivities at intermediate and low temperature. But there are no solid proton conductors working satisfactorily in the gap between, say, 200 and 500° C., as shown in FIG. 1. While the ‘gap’ may seem small in an Arrhenius plot, it covers a most important and desirable range of operating temperatures for both chemical processes and energy conversion processes. Narrowing this gap is of prime interest in the development of proton conductors for practical applications.”
Accordingly, there exists a need for a system having proton conductivity in the “gap” region of FIG. 1. More particularly, there is a need for an electronically insulating proton conductor containing no liquid phase, unlike water in Nafion or H3PO4 imbibed in a matrix, having proton conductivity in the “gap” region of FIG. 2.